Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ he present invention relates to an apparatus and process
for cooling a flow of a molten metal and shaping the cooled metal
in a casting mould.
It is common practice in casting processes that the molten
material is kept at a temperature which is well above its melting
point or liquidus temperature as the case may be in order to ensure
that the material does not solidify prematurely on its way to the
mould or other casting apparatus. As a result of this practice, a
considerable degree of superheat has to be extracted from the
material as well as the latent heat of freezing, so the solidifica-
tion process tends to be slow. This leads to relatively low
throughputs for the casting apparatus, coarsening of the grain
structure, and since cooling in a mould or the like takes place
through the walls, to an inhomogeneous cooling process with con-
sequentially inhomogeneous properties in the casting which is
eventually obtained.
It is an object of this invention to mitigate these
problems of the prior art casting processes by providing a method
for rapidly and controllably coo]ing a moltcn mctal. Accordingly
the present invention provides apparatus for forming shaped
articles from a molten metallic material comprising a mould for
shaping and solidifying the metallic material, a cooling duct for
cooling a flow of the metallic material and directing the flow into
the mould, the cooling duc-t having heat insulatiny side walls, and,
disposed transversely across and within the duct, one or more
static elements adapted to produce turbulence in the material flow-
ing through the duct, at least one of the elements being a thermal
conductor which is further adapted to abstract heat from the
material flowing through the duct and shed the abstracted heat
externally of the duct. The invention further provides a process
for forming shaped articles from a molten metallic material using
the apparatus of the present invention, comprising passing a flow
of molten metallic material through the cooling duct at such a
rate that turbulence is induced in the flow of material, and cast-
ing the flow of material passing out of the cooling duct in the
mould. The mould may comprise, for example, a series of ingot
moulds or a continuous casting machine.
The apparatus and process of this invention are of
especial application to the casting of metallic materials which
exhibit a freezing range of temperatures, for example metallic
alloys~ Most generally, materials as they solidify from the
molten state pass through a condition in which solid and liquid
phases co-exist. For materials which have a sharply defined melt-
ing point this condition occurs at this melting point whilst for
materials which exhibit a freezing range of temperatures a mixture
of solid and liquid material occurs throughout this te~mperature
range, i.e. at all temperatures between the liquidus and the
solidus temperatures. If then a body of rnate~rial in a fully
molten state~, i.e. at a temperature above the liquidus or above its
freezing point as the case may be, is allowed to coo1 to the
liquidus or freezing temperature, solid material will commence to
form and will continue to forrn until the whole body has solidified.
Where the material exhibits a freezing range of temr)eratures com-
plete solidification occurs when the temperature of the whole body
of material reaches the solidus. Where the body of material is
undisturbed solid will form at its periphery from which loss of
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heat takes place, so that, for example, when the molten material
is introduced into a mould, solid begins to form at the walls of
the mould and extends gradually inwards therefrom as heat is
extracted through the walls. By appropriate stirring of the solid/
liquid mixture in the freezing range, the solid which forms can be
obtained as a dispersion of discrete particles suspended in a
maxtrix oE the molten (liquid) material. Such a dispersion of
solid phase in liquid phase is, in the case of a metallic material
which exhibits a freezing range of temperatures, termed a
"metallic slurry" and such dispersions demonstrate advantageous
properties.
In particular, because the slurry is more viscous than a
fully molten material, it is more easy to handle and less liable
to splashing and turbulence on pouring for casting, with the result
that the casting produced is less likely to incorporate air and
will on this account be sounder. Furthermore metallic slurries
are of great advantage as intermediate products since they can have
thixotropic properties which are recreated when the fully solidified
material is reheated to just within the freezing range. In this
state the material has a solid skin and can be treated very much as
if it were still fully solid so that handling of the material is
relatively straightforward, but at the same time it will flow
readily when subject to an applied shearing force. The material
in this condition is therefore most suitable as a feed for a
pressure die casting process and since it does not need to be heated
up to a temperature at which it is fully remelted both a saving in
energy and a reduction in wear and tear on the die casting appara-
tus is achieved.
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It has therefore been conventional practice in producing
metallic slurries to cool a fully molten metallic material to its
freezing point or to within its freezing range as the case may be,
so that solid forms and then to stir the liquid/solid mixture
vigorously for an extended period of time. In this way a slurry
is produced in which the solid material takes the form of a so-
called "degenerate dendrites" which are essentially fragments of
the dendritic primary solid formed as the liquid was cooled and
broken up by the stirring process. Because of the long standing
time and the vigorous stirring which increases the effective rate
of solute transport, the particles in the prior art slurry coarsen
as stirring proceeds, i.e. grow fewer in number but larger in size.
Not only therefore is the prior art process for producing metallic
slurries relatively long and slow but it leads to castings with a
relatively coarse microstructure and having on that account some-
what inferior physical and mechanical properties, particularly in
that a solid with a coarse microstructure exhibits a poor heat
treatment response.
The present process and apparatus are readily applicable
to the production of metallic slurries and their use will mitigate
some or all of the aforementioned problems of the prior art slurry
forming and casting methods whilst retaining to as great a degree
as possible the aforementioned advantageous properties of metallic
slurries.
The process of the present invention has as its primary aims the relatively
rapid extraction of heat from a flowing mass of the molten material which is to
be treated and the simultaneous imparting of a degree of turbulence to the flowing
mass which is sufficient to ensure that the cooling of the flow is homogeneous a-
long the length of the duct. Also the turbulence should be such as to ensure, in
the slurry-making mode, that solid is formed in the mass as a relatively large
number of relatively small particles having a small interparticle spacing and not
as relatively large particles. At the same time the turbulence imparted should
not be so great as to cause the formation of crystals of solid having a degener-
ate microstructure. In slurry making, by causing the solid phase to form in sucha turbulent situation small non-degenerate particles result and as the cooling is
rapid this desirable micro-crystalline structure is retained through to the fully
solidified state, ie there is insufficient time for coarsening of the structure
to occur. At the outlet from the duct the solid phasc typically constitutes from
30 to 65% by volume of the slurry and will generally have a relatively high vis-
cosity. A fully solidified material is produced in the succeeding casting means
by the extraction of the remaining latentheat of fusion of the material.
To assist in achieving substantially homogeneous cooling of the flowing
mass as it moves along the duct it is preferred that the walls of the duct be con-
structed from an insulating material and where the material which is being treat-
ed is metallic it is convenient to use a refractory ceramic material such as the
GC 50 material of the Carborundum Co for the walls of the duct. At the same time
the cooling elements should be made of a material having high heat conductivity
eg a metal or graphite. By using materials for the duct and elements of markedly
differing conductivity it is possible to concentrate virtually all the cooling ef-
fect from the cooling elements and in this way not only can the whole of the flow
be uniformly cooled but also the actual amount of heat abstracted from the flow
can be very precisely controlled. This latter feature is particularly important
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in that it is obviously most desirable to avoid overcooling the material which
might cause it to solidify prematurely in the duct rather than in the casting
means. Such premature solidification is an especial danger in the case where the
material is being supplied to a continuous casting machine since this tends to
draw heat out of the material ahead of it, ie from the material which is still in
the duct due to the chilling effect of the casting machine cooler. This problem
also means that the process of the invention is most suitable applied to materials
which exhibit a freezing range of temperatures.
A further great advantage of the process of this invention is that the
stirring of the flow is achieved without the use of moving machinery. The design
and successful operation of mechanical stirrers in the extreme conditions imposed
particularly by the treatment of molten metals is very difficult to achieve and
the realization that passive stirring means can be employed is a discovery of
considerable value in this art.
The cooling element is most conveniently in the form of a rod of a high
thermal conductivity material. The material preferably also has high erosion re-
sistance: graphite is a suitable material for the cooling rod in these cases.
The rod will generally extend across the duct at right angles to the axis
thereof and may extend outside of the duct in order to provide external cooling
surfaces from which heat extracted from the flowing metal within the duct can be
shed to the outside. For this purpose the rod may have associated means outside
the duct such as fins or vanes for the shedding of heat or alternatively it may
be in the form of a heat pipe or may be hollow to provide a passage-way for the
circulation therethrough of a coolant such as air or water.
In fitting a cooling rod into the duct it will generally be convenient to
drill a pair of aligned holes through the wall of the duct through which a cooling
rod can be passed across the duct. The rod is then cemented to the wall of the
duct by use of a hardenable cement. In the case where metals are to be treated
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a refractory cement such as LDS (Carborundum Co) is most suitably used. A plur-
ality of cooling elements may be provided and may be arranged to lie at angles
to one another when in position in the duct, in order to produce a required de-
gree of turbulence in the molten material flowing along the duct when the appara-
tus is in use.
The number of cooling elements will be chosen to give the required degree
of cooling, taking into account the cooling capacity of each rod, and the amount
of heat which must be extracted from the molten material. This latter is deter-
mined by such factors as the temperature of the material as it is fed into the
duct, the flowrate of the material, the freezing te~perature range values of the
matsrial, the latent heat of solidification of the material, its specific thermal
capacity and of course whether or not it is required to cool the material to just
above its freezing point or solidus temperature or to produce slurry from the ma-
terial. The number and dimensions of the cooling elements in conjunction with
such parameters as the dimensions and inclination of the duct and the flowrate
and viscosity of the material being treated will also determine the shear rate to
which the flowing material will be subjected and it may be necessary in some cases
to make a compromise between the cooling effect and the shearing effect of the
cooling elements. Alternatively, there may be present in the duct additional el-
ements which have no cooling effect but which add to the shearing effect of thecooling elements which are already in place in the duct.
The cooling duct may be formed as a single entity into which cooling ele-
ments can be fitted in the appropriate positions for the intended use of the tube,
or alternatively the duct can be built up as required for a given use from a
series of standard, interconnectable cylindrical parts, some or all of which may
possess in-built cooling elements. By so constructing the duct from standard
items the same parts can be re-assembled to form ducts of different length or
with different numbers or types of cooling element if requirements change. Con-
siderable flexibility is thereby introduced, and additionally access for cleaning
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or repair when required is enhanced since the duct can be readily dismantled to
its component parts.
The molten material which is to be cooled or converted to slurry is prefer-
ably held in an insulated vessel which may have a heater so that the charge of
material within it can be maintained at a suitable temperature. Ideally in theory
the temperature of the material should be just in excess of its freezing point or
of the top limit of its freezing range, ie justabove the liquidus temperature
where the material exhibits a freezing range, but as it is difficult to keep a
large body of material at uniform temperature, especially a high one where the ma-
terial is metallic, and to avoid any danger of the material solidifying in thevessel or even more seriously in the outlet leading to the cooling duct it is of-
ten necessary to maintain the material at a temperature which is 30C or more in
excess of the liquidus temperature or freezing point. Furthermore keeping the
molten material well above its liquidus temperature will help to prevent "sludging"
ie the gathering of impurities at the bottom or top of the melt.
Control of the flowrate of material from the holding vessel may be effected
simply by the provision of a relatively narrow outlet from the holding vessel.
Alternatively a valve may be used to control the outflow of the molten material to
the duct. Where a throttling connecting pipe is used and particularly when the
material in the holding vessel is at a temperature which is not greatly in excess
of its freezing point or liquidus temperature as the case may be, then the connect-
ing pipe should preferably be as short as possible and may be insulated or even
heated in order to ensure that the material does not tend to solidi~y within it.
It is convenient during a casting run to maintain the flowrate constant so
that the cooling duct may operate under steady state conditions, as this ensures
that the output from the duct is homogeneous with time, and that a consistent cast
product is obtained. To do this it may be convenient to arrange that the vessel
for the molten material provides a constant head of material. This may be done
either automatically such as by provision of a weir or by a controlled pour of
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material from a relatively large vessel into a smaller header vessel, or by pro-
viding a suitable pump which works at a steady rate. A pump is particularly ap-
propriate for use where the desired flowrate could only be met by providing a very
large head of molten material which may be physically inconvenient. The cooling
duct may be arranged to lie vertically or horizontally or at any intermediate in-
clination as is convenient considering the working space and height available.
After passing through the duct the cooled material may be led immediately
into a fixed mould of a conventional kind for ~he material being cast or into the
"cooler" die of a continuous casting machine from which it is withdrawn as a solid
bar or rod in the conventional manner.
The combined stirring and cooling of the melt in the cooling duct when suf-
ficient to form a slurry therein results in a significant proportion of the latent
heat in the material being removed without the possibility for large dendritic
crystals to grow so that where the initial temperature of the melt is properly
controlled there will be a large number of small particles of solid material cre-
ated in the melt. As a result of this removal of heat from the material before it
enters the mould or die, the material once it has entered the mould or die cools
rapidly to give a solid which has a fine equiaxed grain structure. Not only does
this lead to an improved product but also the use of the cooling duct effectively
as a means for cooling a molten material in advance of its passing into the cast-
ing apparatus enables that apparatus to achieve a much higher throughput of mate-
rial than is possible when the supply to the casting apparatus is fully molten as
is conventional.
An embodiment of apparatus according to the invention and suitable for the
casting of metallic materials will now be described by way of example only and
with reference to the accompanying drawings in which:
Figure 1 shows the apparatus in sectional elevation;and
~ igure 2 is a sectional elevation on the line II-II of Figure 1, through a
slurry-making duct in the form of a tube forming part of the apparatus of Figure 1.
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The apparatus of Figure 1 comprises a vessel 1 for holding molten material,
the vessel having an outlet 2 through which it communicates with the upper end of
a short downpipe 3. At its lower end the downpipe opens into a duct in the form
of a slurry-making tube 4. The slurry-making tube is substantially horizontally
disposed and opens at the end farthest from the downpipe into a die 5 forming part
of a continuous casting machine.
The holding vessel 1 is heated by radiant elements 6 so as to maintain its
charge at the desired temperature and is enclosed in a chamber shown somewhat
schematically at 7 to prevent it being subject to draughts. The downpipe too is
preferably heated at least initially during a casting run so as to prevent the
molten material first entering the downpipe from freezing. A coiled heater ele-
ment 8 may be used for this purpose.
The slurry-making tube 4 is provided with a number of transversely disposed
rods 9 passing through apertures 10 drilled in the tube walls (Figure 2). The
rods are sealed into the apertures by a layer of cement 11. The rods are hollow
having a passageway 12 through them through which a coolant can he circulated by
means not shown in the drawings. Alternate rods are mutually disposed at right
angles.
The cooler die 5 of the continuous casting machine comprises an annular
graphite block 13 aligned with the end of the slurry-making tube and into which
the tube end projects making a tight fit with the block. The slurry-making tube
is held firmly to the block of the continuous casting machine by means of tie bars
17 secured to the block 13 at one end and at their other ends passing through ap-
ertures in the end plate 4A of the slurry-making tube and carrying nuts on a
threaded portion which nuts can be tightened against plate 4A. To allow for line-
ar expansion of the tube in use springs are provided between each nut and the face
of the end plate 4A. The block 13 of the continuous casting machine is surrounded
by an annular water jacket ~4 eg Gf copper, shrink-fitted to the graphite block
for good thermal contact and provided with inlet and outlet so that a stream of
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cooling water 15 can be circulated therethrough. The continuous casting machine
also has a pair of pinch rollers 16, 16' arranged in line with the aperture in the
die 5 and driven by an electric motor not shown.
In use, the cooling water circulation through the jacket 14 is started and
a starter bar 18 inserted into the aperture in the die 5. The rear end of the bar
engages between pinch rollers 16,16'. The downpipe heater 8 is switched on and
the pipe heated up to an appropriate temperature. Likewise it may be convenient
to preheat the cooling/stirring rods using heated wires passed through the rods
preliminary to a casting run. These are withdrawn and the cooling means (if any)
connected up just prior to allowing the material to be cast to enter the slurry-
making tube. A molten metal alloy 19 which exhibits a freezing range of tempera-
ture is poured into the holding vessel 1 and after a delay which is calculated or
measured in a calibration run, the supply of coolant (if any) to the cooling/stir-
ring rods is commenced and the rollers 16, 16' are started to turn, thus drawing
the starter bar out of the die and away from the slurry-making tube. As the mol-
ten metal passes down the down tube 3 and along the slurry-making tube 4 it is
cooled and solid commences to form in the liquid metal, so that a metallic slurry
is generated. As the metallic material passes in this state into the cooler die
of the continuous casting machine complete solidification results and the solid-
ified material 20 attaches to the starter bar 18 and is steadily withdrawn therebyuntil the solidified material itself is engaged between the pinch rollers 16, 1~'.
After this point is reached the starter bar may be detached from the solid mater-
ial though this is preferably done by first cutting off the end portion of the
solidified material together with the bar and then either melting that material
off the bar or otherwise removing it.
As an illustration of actual operating conditions for the process of the
invention, an apparatus of the type generally described and illustrated hereinbe-
fore which was designed for the casting of an aluminium alloy by way of a slurry
will now be described by way of example only. The apparatus was designed to
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produce continuously cast aluminium bar at a rate of 75 Kg/hour using as a part
of the apparatus a continuous casting machine.
Molten aluminium alloy LM4 ~B.S.1490) was supplied from a holding vessel 1
where it was maintained at a temperature of 660C. Alloy of this type has a
freezing tempature range of from 611C to 520C. The molten alloy was allowed to
pass freely through the outlet 2 in the bottom of the vessel 1 and down the unin-
sulated downpipe 3 which was 440 mm long and of 18 mm internal diameter into oneend of an horizontally disposed slurry-ma~ing tube 4. At the bottom of the down-
pipe the temperature of the alloy was measured as 640C. The slurry-making tubewas constructed of GC50 refractory ceramic material which is a silica fihre-
strengthened alumina composition and was 425 mm long with an internal diameter
~id) of 38 mm and a minimum wall thickness of 29 mm. Disposed across the tube
were ten hollow graphite cooling rods, each 96 mm long and with 5 mm id and 15 mm
od. The rods were disposed perpendicular to the axis of the tube with alternaterods at right angles to each other and were each spaced apart longitudinally of
the tube by 20 mm. The rods were connected to an air supply line so that a con-trolled volume of air could be blown through them, by means of flexible hoses
terminating in copper tubes which fitted tightly into the rod ends. In the run
being described air was blown through the last two rods only, through the ninth
rod at a pressure of 10 psi and through the tenth rod at a pressure of 30 psi,
and the temperature of the a]loy measured just down-stream of the tenth rod was
592C. The slurry was fed to a continuous casting machine designed to produce
bar of diameter 69 mm at a rate of 110 mm/minute with a water-cooled graphite die.
In the present run the casting rate was 220 mm/minute, but in other runs castingrates in excess of 300 mm/minute have been achieved.
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The same apparatus was used to cast aluminium alloy LM24, (B.S.1490) the
temperature of which in the holding vessel was 639C and which has a freezing
temperature range of from 5~0C to 520C. Cooling was effected by passing air
through the final three cooling rods at 5 psi. The alloy casting rate in this
case was 200 mm/minute.
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